1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system, and more particularly to a binding processing mechanism intended for sheets such as sheets of paper after image forming.
2. Description of the Related Art
On sheets such as sheets of paper printed out by an image forming apparatus such as a copying machine and a printer or a printing machine, post-processing such as a binding process by a stapler to bind a part of a given number of sheets in the condition in which the sheets are gathered may be performed other than being ejected from the image forming apparatus. As an apparatus for this purpose, a sheet processing apparatus connected to an ejecting unit of the image forming apparatus is used.
What performed by the sheet processing apparatus includes not only a binding process aimed for a corner of the leading end of the ejected sheets in the ejecting direction or one side thereof and a saddle stitching process that binds the center of the sheet of paper, but also a folding process that doubles up the saddle-stitched sheets at the saddle-stitched portion for bookbinding.
As a configuration to perform folding work, a known configuration includes a conveying roller that conveys a sheet being a target of the folding work, and a stopper member against which the leading end of the sheet is pressed and stopped (for example, Japanese Patent Application Laid-open No. 2004-284742).
In this configuration, a process of folding a buckling-deformed portion of the sheet, which is formed when the sheet is further conveyed in the condition in which the leading end of the sheet abuts the stopper member, when clamping and conveying the buckling-deformed portion by the conveying roller is repeated.
Meanwhile, as a process prior to performing the folding work, a creasing process may be performed.
As one of the configurations used for creasing work, a configuration is disclosed in which a member having a projecting tooth and a pedestal facing the member are disposed across the conveying path of the sheet of paper (for example, Japanese Patent Application Laid-open No. 2011-057363).
In this configuration, moving the member having the projecting tooth towards the pedestal creases the sheet.
When creasing is performed, the creasing positions of respective sheets may deviate from one another if the position of the creasing work is not appropriate. If the positions of creasing deviate from one another, the folding positions of the sheets deviate and the advantageous effect of creasing is not sufficiently obtained when the folding work such as half hold is performed. Furthermore, when the sheets are matched at the creasing positions that deviate from one another, there arises a problem that the ends of the sheets after folding work do not align with one another.
Consequently, in the configuration disclosed in Japanese Patent Application Laid-open No. 2011-057363, the configuration is disclosed in which the conveyance of the sheet is stopped before the sheet reaches the folding unit in the condition in which the sheet is clamped at the part near the leading end of the sheet positioned downstream in the sheet conveying direction, and rotates the conveying roller on the upstream side in the sheet conveying direction in reverse to exert a tensile force to the sheet so as to eliminate the deflection.
In the configuration disclosed in Japanese Patent Application Laid-open No. 2011-057363, however, being not able to exert the tensile force corresponding to the amount of deflection of the sheet causes the following flaw.
That is, the deflection occurs when the leading end of the sheet is pressed against and stopped by a pair of rollers positioned on the downstream side in the conveying direction. Consequently, the deflection remains at the position before the position clamped by the pair of rollers when the sheet is conveyed until the downstream side of the sheet in the conveying direction is detected. The amount of the deflection varies greatly depending on the curl condition (magnitude of curl) and the bending stiffness of the leading end of the sheet of paper.
Consequently, even when the conveying roller is rotated in reverse to generate the tensile force to the sheet, it may result in that the deflection remains not eliminated, that the durability of the conveying rollers deteriorates due to an increase in the amount of slip, or the like when the amount of reverse rotation is far different from the amount of actual deflection.
As for the amount of deflection, whether to perform skew correction also makes it vary greatly. More specifically, in the skew correction in which conveying is continued on the upstream side in the conveying direction while the sheet is pressed against and stopped by a pair of rollers, the amount of deflection is likely to increase unlike when the skew correction is not performed. Thus, unless the correction corresponding to the amount of deflection is performed properly, the position of creasing may not be calculated accurately.
In view of the above, there is a need to provide a sheet processing apparatus and an image forming system that can make the creasing position of the sheet even between the sheets with a simple configuration regardless of the difference in the amount of deflection.